Field of the Invention
The present invention relates to an image forming apparatus, such as a printer and a digital multifunction imaging apparatus.
Description of the Related Art
Regarding image forming apparatuses especially for business use, there exists a demand for a higher level of image property (i.e., color stability and reduction in image roughness) of images printed on a sheet as a recording material recently. The image property at a high level is demanded in a wide range of media, from thin paper of about 50 gsm of basic weight used for, for example, printing flyer or direct mail, to a cardboard of about 400 gsm used for package printing, for example.
Especially in an electrophotographic image forming apparatus, in order to stably transfer a printing image from an image bearing member to a sheet, images are formed independently in each color and the formed color images are registered on an intermediate transfer member (a belt or a roller) (primary transfer). The registered image is then secondarily transferred to the sheet (a secondary transfer system).
Since a disturbance factor caused by the physical properties of the sheet during registration of the colors can be eliminated by the intermediate transfer member, the secondary transfer system is advantageous in easily obtaining stable image transferability compared with a direct transfer system in which colors are registered directly on the sheet.
In order to enable an image forming apparatus of the secondary transfer system to print on various types of sheets, an external secondary transfer belt is desirably used at a portion at which an image is transferred from the intermediate transfer member to the sheet. This is because the external secondary transfer belt provides high transferability also when a sheet with unevenness on its surface, such as embossed paper, is used, or provides improved sheet conveyance property in the secondary transfer portion since it is possible to stick the sheet to the external secondary transfer belt with bias charge during transfer.
When the external secondary transfer belt is used, since the sheet substantially sticks to the external secondary transfer belt, the conveyance speed of the sheet is substantially the same as the surface velocity of the external secondary transfer belt. Since the sheet is conveyed sticking to the external secondary transfer belt, the conveyance speed of the sheet is determined by the surface velocity of the belt with substantially small fluctuation. Therefore, stable image magnification is obtainable by driving the external secondary transfer belt at a constant speed.
The sheet conveyance speed is stabilized by driving the external secondary transfer belt at a constant speed, however, a speed difference is caused due to a small difference in the curvature radius inside a secondary transfer nip between the intermediate transfer member and the sheet surface. In order to make the sheet surface velocity the same as the surface velocity of the intermediate transfer member, the driving speed of the external secondary transfer belt needs to be controlled optimally depending on the type of the sheet.
In the related art which employs a secondary transfer roller, a driving source for an intermediate transfer belt and a driving source for the secondary transfer roller are prepared independently. The sheet conveyance speed may vary depending on the thickness of the sheet. Therefore, adjusting the secondary transfer roller speed depending on the thickness of the sheet is known. (see Japanese Patent Laid-Open No. 2008-281931).
In a secondary transfer system which employs an elastic material on a surface of the intermediate transfer medium and a non-elastic material as an external secondary transfer belt, a speed difference between an intermediate transfer belt surface and a sheet surface may be caused due to the existence of a sheet material in the secondary transfer portion. As the sheet enters a secondary transfer nip, the shape of the secondary transfer nip changes, and the speed relationship between the intermediate transfer belt and the secondary transfer portion in the secondary transfer nip changes from the speed relationship before the sheet enters.
It is known that the optimal surface velocity setting of the external secondary transfer belt necessary to provide the optimal image quality for each sheet type cannot be determined only by the thickness of the sheet. Image transfer quality in the secondary transfer nip is influenced greatly by how uniformly the image is transferred to the sheet surface. The slidability between the intermediate transfer member and the sheet in the secondary transfer nip is affected by the slidability of the sheet surface. That is, the optimal external secondary transfer belt speed setting of paper with a low friction coefficient, such as regular paper, and the optimal external secondary transfer belt speed setting of paper with a high friction coefficient, such as gloss coated paper are different from each other depending on the surface properties of the sheet.
If the speed relationship between the intermediate transfer belt and the secondary transfer portion in the secondary transfer nip is adjusted, color misalignment may occur. To avoid occurrence of color misalignment, image formation is interrupted and color misalignment correction control (hereafter, referred to as automatic registration control) is conducted in the related art. Automatic registration control produces, however, downtime. Some users may give priority to productivity over image quality, and such downtime causes problems in satisfying the user demand of image quality or productivity. There may be a configuration in which the automatic registration control is not conducted when the user gives priority to productivity. In an example, the speed relationship between the intermediate transfer belt and the secondary transfer portion cannot be adjusted even if the paper type is changed. In this case, since the speed is uncontrollable, the demand for image quality cannot be satisfied.